Revert "zfs: add btree.c to libzfs"

This reverts commit b4fdb5c236.

cddl/lib/libzfs/Makefile

@@ -5,7 +5,6 @@
 .PATH: ${ZFSTOP}/include
 .PATH: ${ZFSTOP}/module/zstd
 .PATH: ${ZFSTOP}/module/zstd/lib
-.PATH: ${ZFSTOP}/module/zfs
 
 PACKAGE=	zfs
 LIB_PACKAGE=
@@ -53,7 +52,6 @@ USER_C += \
         libzfs_zmount.c
 
 KERNEL_C = \
-	btree.c \
         cityhash.c \
         zfeature_common.c \
         zfs_comutil.c \